Card Balances and Other Loans - Performance (Details) - USD ($) $ in Millions	Sep. 30, 2026	Sep. 30, 2025
Current
Financing Receivable, Modifications [Line Items]
Account balances	$ 3,138	$ 2,891
30-89 Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	208	224
90+ Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	77	81
Card Balances | Current | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	2,003	1,863
Card Balances | Current | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	1,023	937
Card Balances | Current | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	16	11
Card Balances | 30-89 Days Past Due | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	118	128
Card Balances | 30-89 Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	82	87
Card Balances | 30-89 Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	2	4
Card Balances | 90+ Days Past Due | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	48	47
Card Balances | 90+ Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	26	31
Card Balances | 90+ Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	2	2
Other Loans | Current
Financing Receivable, Modifications [Line Items]
Account balances	96	80
Other Loans | 30-89 Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	5	5
Other Loans | 90+ Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	$ 1	$ 1